Automotive Electrical Diagnosis: Chasing Down 5 Volt Reference Issues

[mrrsm]

It's no *SECRET* that my preference in Automotive Diagnostics leans heavily towards using Automotive Oscilloscopes. But... Their USE is invariably applied in conjunction with The USE of Other Basic Hand and Diagnostic Tools such as a Lighted Probe and either a DMM or a DVOM.

The Big Take Away here is that this effort must always include a Basic Understanding of the Systems involved, and their Wiring Diagrams. You don't necessarily NEED any Exotic Equipment to be a Good Diagnostician... until you are trying to VISUALIZE problems in any Electrical Paths that are So Brief and So Transient that the typical DMM will NOT be able to catch any useful views of these events...as Wave Forms... even if your DMM is fitted with a Min-Max Feature.

All That Said... Folks like @TJBaker57 and @budwich will often dial right in on the Who, What, Where, Why and When of certain Electrical issues because they KNOW what they are doing within the realm of Auto Diagnostic Step BY Steps. They have proven and often explained that Understanding the Basics of Electricity and using Common Sense and Good Judgement will more than likely Win The Day and nail down what the REAL issues are in finding and fixing such problems.

THESE Videos are Universal Examples in direct support of what you can always find in all of their Threads and Posts at GMT Nation based upon this premise

"ELECTRICITY IS THE SAME ...NO MATTER THE MAKE OR MODEL OF THE VEHICLES INVOLVED..."

 

[mrrsm]

If you maintain a "Mechanic's Multi-Media Library" , having some PCM Connector and PCM Pin-Out Diagrams highlighted to Identify ALL of the "5 Volt Reference Voltage Feeds" in advance will reduce the Diagnostic Time it takes to isolate them all.

The recipient Modules must be Disconnected ...One at a Time ...until the one causing the Short To Ground 5 Volt Reference Voltage *Plumps* back up to Five (5) Volts on your DMM or DVOM set to DC Volts as soon as IT gets unplugged. Here are some Example Images:

 

[JayArr]

Another handy 'tool' is a PCM pin and 10 feet of wire, I call it a diagnostic bypass wire. It costs about $1 to make. Use MRRSM's method until you think you've found the bad sensor then remove the OEM pin and insert your diagnostic wire pin in the PCM connector and hook it back onto the PCM.

Remove the wiring connector from the sensor under suspicion and remove the 5V wire from that connector and insert the other end of your diagnostic wire. What you have now done is bypass the wiring harness of the vehicle.

If the sensor still pulls the 5V low then the sensor is bad. Replace and re-test.

If the 5V now stays at 5V and the vehicle works, the problem is somewhere in the wiring loom.

If it's the loom then you can tear into the wiring and look for the problem but I'm pragmatic, I just run a new wire and tie-wrap and tape it to the outside of the loom. You can run a new wire for $1 and be done in 10-20 minutes but unwrapping a wire loom to find a chaffed wire could take a whole afternoon. I've got other things to do and don't mind extra wires if they give me my Saturday afternoon back to me.

I'm an Electronics Engineering Technologist first and an automotive weekend warrior second so I've had the advantage of electronics troubleshooting experience overlap.